The present invention relates to a silent chain used in combination with sprockets, and more particularly to such a silent chain which is capable of reducing noises and wear produced due to meshing engagement between the chain and the sprocket.
Silent chains include guide link rows and intermediate link rows alternately connected together in an endless fashion. In a typical example of such silent chains, the guide link rows each have a pair of guide plates and at least one link plate disposed between the guide plates. The guide plates and the link plate each have a pair of pin holes spaced in the direction of travel of the silent chain. The intermediate link rows each have two or more link plates numbering one more than the number of the link plates in each guide link row. The link plates, like the link plates in the guide rink rows, each have a pair of spaced pin holes.
Each of the guide link rows and an adjacent one of the intermediate link rows are articulately connected together by a connector pin inserted through the laterally aligned pin holes of the link plates and guide plates. The connector pin may be a round pin or a pair of rocker joint pins.
The silent chain is used for transmitting power between a driving sprocket and a driven sprocket through meshing engagement with the sprockets. In a practical application, the silent chain is wound around a crank sprocket and a cam sprocket of an automobile engine, or around the output shaft of an automobile transmission and the input shaft of a transfer unit. Consideration will be first given to the method of producing the link plates and the structure of a surface formed by a punching operation. The link plates are usually formed by punching a metal blank into a desired shape and configuration. In the punching or other blanking operation, a die-cut surface of the link plate unavoidably has a smooth shear surface and a rough rupture surface. According to a conventional design concept, special weight is given to the way of increasing the proportion of the shear surface with a view to improving the strength of the silent chain. This is partly because a high proportion of the rupture surface may cause cracking, and partly because a narrow shear surface results in an increased surface pressure on tooth flanks of the link plate, leading to deterioration of the wear resistance and fatigue strength of the silent chain.
Next, consideration will be given to the behavior of the silent chain at the time of meshing with a sprocket. Regardless of the difference in type of the link rows, meshing starts to occur between flank surfaces of the link plates and tooth flanks of the sprocket. In one type of silent chain, at the point where the silent chain starts to engage the sprocket, an outer flank of one link plate located at the rear side in the travel direction first comes into contact with a tooth of the sprocket. In another type of silent chain, at the point where the silent chain starts to engage the sprocket, an inner flank of one link plate located at the front side in the travel direction first comes into contact with a tooth of the sprocket. In any case, when the silent chain starts to mesh with the sprocket, noises occur due to collision between the flank surface of each link plate and the tooth flank of the sprocket.
As discussed above, the rigidity of the link plate increases with the proportion of the shear surface. However, increasing the proportion of the shear surface will give rise to a problem that collision sounds produced by engagement between the link plates and the sprocket are intensified. In automobile engines and automobile transmission where compatibility of durability and quietness is a major requirement, sufficient consideration must be given to the noise suppressing measure as well as to improvements in strength. Through observations of the dynamic behavior of a silent chain, the present inventors uncovered the fact that the area of the flank surface which takes part in the improvement in the strength of the silent chain is not necessarily the same as the area of the flank which takes part in the emission of noises from the silent chain. Rather, these areas are different from each other. Stated in detail, an area of the outside flank surface of each link plate, which is located near a chain pitch line, receives power from the tooth flanks of the sprocket, and an area at which the link plates start to engage the sprocket is a portion of the inside or the outside flank surface located near the tip of a tooth of the sprocket.
In completing the present invention, account was taken of the fact that, even when the ratio of the shear surface to the rupture surface is differentiated between the area at which a flank surface of each link plate starts to engage the surface of a tooth of the sprocket, and the area at which the flank surface of the same link plate receives power from the surface of the tooth of the sprocket, a sufficient chain strength can be maintained, provided that the power-receiving area has a sufficiently large shear surface.
It is accordingly an object of the present invention to provide a silent chain which is capable of reducing collision noises occurring between the flank surface of a link plate and the tooth flank of a sprocket when the silent chain starts to engage the sprocket, and also is able to maintain the desired wear resistance and fatigue strength of the link plate after completion of meshing engagement between the silent chain and the sprocket.
According to the present invention, there is provided a silent chain for use with a sprocket, comprising: a plurality of interleaved rows of link plates connected together in an endless fashion, each of the link plates having a pair of teeth, each tooth having a flank surface, a mesh start area at which the tooth starts to mesh with a tooth flank of the sprocket, and a power-transmitting area at which the tooth receives power from the tooth flank of the sprocket. The mesh start area and the power-transmitting area are located at different positions on the flank surface. The mesh start area has a smooth contact surface and a non-contact surface arranged side by side in the direction of thickness of the link plate. The non-contact surface is set back from the smooth contact surface, and the smooth contact surface is smaller in width than the non-contact surface. The power-transmitting area has a smooth contact surface and a non-contact surface arranged side by side in the direction of thickness of the link plate. In the power-transmitting area, the non-contact surface is set back from the smooth contact surface, and the smooth contact surface is greater in width than the non-contact surface.
With this arrangement, when the silent chain starts to engage the sprocket, the inside flank surface or the outside flank surface first moves into contact with the tooth flank of one tooth of the sprocket at a position adjacent to the tooth tip. In the mesh start area at which the inside or outside flank surface start to move into meshing engagement with the tooth flank of the sprocket, the rate of occupancy of the smooth contact surface is relatively low and hence able to reduce collision sound produced between the smooth contact surface and the tooth flank of the sprocket. The mesh start area does not take part in the power-transmitting operation and hence is freed from the problem of wear.
As the silent chain further advances along the periphery of the sprocket, the same tooth flank of the sprocket comes into contact with a different portion of the tooth flank than the portion including the mesh start area. When the silent chain has established complete mesh with the sprocket, the tooth flank of the sprocket is in contact with a power-transmitting area completely different from the mesh start area and thus transmits power to the power-transmitting area. In the power-transmitting area, the occupancy rate of the smooth contact surface is relatively high, so that the power-transmitting area is able to perform a power-transmitting operation without involving undue surface pressure. The power-transmitting area has high wear resistance and fatigue strength, which leads to a long service life of the silent chain.
In one preferred form of the invention, each of the teeth has an inside flank surface and an outside flank surface merging at a tip of the tooth, and the rate of occupancy of the smooth contact surface in the outside flank surface increases gradually in a direction from the tip of the tooth toward a base of the tooth. This arrangement is particularly useful when embodied in a silent chain of the type wherein the mesh start area and the power-transmitting area are both provided on the outside flank surface.
The rate of occupancy of the smooth contact surface in the inside flank surface may be lower than the rate of occupancy of the smooth contact surface in the outside flank surface. This arrangement is particularly useful when applied in a silent chain of the type wherein the inside flank surface first comes into contact with the tooth flank of the sprocket, and the outside flank surface is then engaged in the power-transmitting operation. The inside flank surface, which is not engaged in the power transmitting operation, has the smooth contact surface at a low occupancy rate and, hence, is able to reduce noise and has an improved degree of durability and fatigue strength. The rate of occupancy of the smooth contact surface in the inside flank surface may uniform throughout the length of the inside flank surface, in which instance the smooth contact surface and the non-contact surface in the outside flank surface may be in phase with, or 180xc2x0 out of phase with, the smooth contact surface and the non-contact surface in the inside flank surface. Alternatively, the rate of occupancy of the smooth contact surface in the inside flank surface may increase gradually in a direction from the tip of the tooth toward the base of the tooth. The smooth contact surface in the outside flank surface may be symmetric to the smooth contact surface in the inside flank surface with respect to the tip of the tooth, in which instance the smooth contact surface and the non-contact surface in the outside flank surface may be in phase with, or 180xc2x0 out of phase with the smooth contact surface and the non-contact surface in the inside flank surface.
The rate of occupancy of the smooth contact surface in the power-transmitting area may be more than 40%, and preferably more than 70%. The rate of occupancy of the smooth contact surface in the mesh start area is preferably in the range of 7 to 15%.
The link plates in one row and the link plate in another row may have different rates of occupancy of the smooth contact surface. These rows of link plates may be arranged either in a regular pattern or in a random pattern in the longitudinal direction of the silent chain. This arrangement is highly effective in reducing noise caused by resonance of periodic sounds.
Similarly, the link plates in each row may have different rates of occupancy of the smooth contact surface. The link plates with different rates of occupancy of the smooth contact surface may be arranged either regularly or at random in the widthwise direction of the silent chain. This arrangement contributes to the reduction of noise.